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[Reprinted from American Chemical Journal, Vol, VI, No. 5.] 
Contributions from the Sheffield Laboratory of Yale College. 

XXL— ON THE RELATIVE DIGESTIBILITY OF FISH 
FLESH IN GASTRIC JUICE. 

By R. H. CHiTTENDErj AND Geo. W. Cummins. 

The value of food as nutriment depends primarily upon the 
presence in suitable quantity of elements, or combination of ele- 
ments, capable of supplying the needs of the body ; coupled with 
this, however, is the ease with which the food stuff in question can 
be rendered available by the system for its wants. This, or in 
other words its digestibility, constitutes a very important item in 
determining the true nutritive value of any food. If, of two foods 
possessing a like chemical composition, one be more easily diges- 
tible, that one, though containing no more available nutriment than 
the other, is in virtue of its easier digestibility more valuable as a 
food stuff and in one sense more nutritious, as well as more 
economical for the system. 

Both chemists and physiologists have appreciated the impor- 
tance of all data relative to the nutritive value of foods. But 
hitherto nearly all work in this direction has been confined to a 
study of chemical composition, and only occasionally to digesti- 
bihty. The mere fact, however, that a substance contains a certain 
percentage of nitrogen is not alone sufficient. We need to know 
in addition, not only how much of the nitrogen passes through the 
body unabsorbed, thus indicating how much is ordinarily available 
for nutriment, but we need to know likewise how long the food 
stuff remains in the stomach, how quickly it is acted upon by the 
digestive juices, and, finally, how much passes out undigested ; 
points of great importance to the healthy system, but still more 
so to the system weakened by disease. 

There are two ways of determining the digestibility of a food 
stuff in gastric juice. One consists in the introduction of a weighed 
amount of the substance into the stomach of a man or animal 
through a fistulous opening, and noting the length of time required 
for its solution; the other, in the use of an artificial gastric juice by 
which the amount of substance capable of being dissolved and 
digested in a given time can be quantitatively ascertained. The 



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first of these methods was made use of by Dr. Beaumont in his 
celebrated experiments on the Canadian, Alexis St. Martin, about 
1830, and has been employed many times since with animals by- 
other workers. While it would appear in some respects to be the 
better method, there are reasons why it is not so advantageous as 
the other, mainly because it is not capable of showing such small 
differences, it is not free from nervous influences and personal idio- 
syncrasies, and, lastly, it is less convenient. The method with 
artificial gastric juice, on the other hand, admits of the conditions 
being the same in each case, and since the digestion of a food is 
by itself simply a chemical process, it would seem better in a de- 
termination of digestibility that the process be shorn of all those 
conditions, natural or otherwise, which tend to interfere with the 
purely chemical action of the digestive juice. 

Few experiments appear to have been made on the digestibility 
of fish ; this is the more strange when we consider what an impor- 
tant item of food fish constitutes, particularly along our seaboard. 
Yet the idea is prevalent, based apparently on general grounds, 
that fish flesh is not easily digestible. Thus Maly' mentions that ) 
" fish flesh is difficult of digestion, although the reason is not 
known." Still, as Voit^ remarks, " nothing certain is known regard- 
ing the digestibility of different kinds of flesh, although much is 
said concerning it. Probably digestibility is in part dependent 
upon the nature of the fat present and the manner of its distribu- 
tion ; thus the presence of a difficultly fusible fat with considerable 
stearin would tend to hinder digestibility (as in mutton) ; the same 
thing probably occurs when the contents of the sarcolemma are 
permeated with much fat (as in the lobster an/5 eel.)" This state- 
ment at once suggests the probability of great variation in the 
digestibility of the flesh of any one species, dependent on a large 
number of conditions, which, in the case of fish particularly, are 
somewhat difficult of control ; thus age, sex, food, period of spawn- 
ing, length of time they have been preserved,- are a few of the 
many natural conditions which would tend to modify the digesti- 
bility of the flesh and render generalisations from even a large 
number of results somewhat uncertain. 

Still, as no systematic experiments appear ever to have been 
tried with fish flesh, we have attempted to obtain some positive 
results concerning the relative digestibility of the more common 

1 Hermann's Handbuch der Physiologic 5, 112. ^Ibid. 6, 447. 



edible species, as well as the general digestibility of fish as com- 
pared with beef, veal, lamb, etc. 

The Method Eviployed. 

The gastric juice. — For reasons already given, artificial diges- 
tion was chosen as the best adapted for the purpose, and with this 
end in view a gastric juice was needed which should be both con- 
stant in composition and activity during the length of time required 
for trying the experiments. A large quantity of so-called " pure 
pepsin " ' was obtained, thoroughly sampled, and then placed in a 
tightly stoppered bottle and kept in a cool, dry place to prevent 
change. The acid used was pure hydrochloric of exactly 0.2 per 
cent, strength. From this material fresh gastric juice was made 
for each series of experiments, 5 grams of the pepsin being dis- 
solved in I litre of the dilute acid. This furnished a digestive 
mixture of suitable strength, and, as subsequent experiments 
showed, well adapted to the purpose. 

Preparatio7i of the flesh. — In order that a fair sample of the flesh 
might be obtained in each case, 100 grams, freed from tendons, fat, 
skin and bones, were weighed off and finely divided by chopping. 
Small portions of the sample tissue were then taken for a determi- 
nation of the amount of solid matter,'- and then two portions of 
20 grams each were weighed out to determine the digestibility. 
These t\\U latter portions were placed in small porcelain mortars 
covered with watch glasses, and then set into a steam-bath heated 
by a large gas flame for 30 minutes. This bath was a small 
copper oven, on the bottom of which was a layer of water i or 2 
inches in depth, while some distance above this was placed a per- 
forated plate upon which the dishes were set, the whole provided 
with a tightly fitting cover, with a small outlet for the escaping 
steam. Heated in this for half an hour the fish or meat was thor- 
oughly and evenly cooked without loss of any extractives, and 
being in mortars, the tissue after steaming could be ground up fine 
without loss. The flesh was then ready for digestion. 

The digestion. — As already stated, two separate or duplicate de- 
terminations were made of each sample. Each portion of 20 grams 
was placed in a beaker with 200 cc. of the standard gastric juice, 
covered with a watch glass and set into a bath heated at 38-40° C. 

■ Manufactured by Henry Thayer & Co. 

"^ Determined by simply drying at ioo° C. until of constant weight. 



for 22 hours with occasional stirring. This bath consisted of a 
metal box with a movable cover, and having about midway of its 
height a perforated plate upon which the beakers were placed. The 
bath was filled with water to such an extent that the beakers were 
immersed about half an inch. The space above, when the bath 
was closed, was of course saturated with aqueous vapor, and thus 
any evaporation of the contents of the beakers was prevented. 
The temperature was kept quite constant by a small gas flame, and 
the extreme variations were not more than 35-42° C, these occur- 
ring only during the night and early morning. The length of time 
the mixtures were heated, viz. 22 hours, was no longer than was 
necessary to insure accurate and concordant results. In an artifi- 
cial digestion the accumulation of the products formed tends to 
retard the action of the fluid, but in no case were our results im- 
paired by saturation of the digestive mixture, for that this could 
never have occurred ordinarily is plainly shown by the large 
amount of blood fibrm dissolved by the gastric juice in a trial 
experiment. The addition of larger amounts of flesh, moreover, 
in the case of fish, 30-40 grams, simply diminished the digestive 
action. 

Deter viinaiion of the amount digested. — This can be accom- 
plished by either weighing the undigested residue, or by determin- 
ing directly the amount dissolved. In a recent work by Jessen' 
on the influence of different modes of preparation on the digesti- 
bility of meat, the former method was used. Unless the amount 
of water contained in the meat experimented with, however, is 
determined, a very decided error may be introduced. Thus Jessen 
found by experiment with frogs' legs' that 2 grams of the raw flesh, 
introduced into the stomach of a dog, required on an average 4.46 
hours for digestion, while the same amount of beef, similarly pre- 
pared, required on an average 5.58 hours, and thus from this ex- 
periment the relative digestibility of the two would be as 84 : 100. 
Our experiments, however, show quite a different result, easily 
explained by a determination of the percentage amount of solid 
matter in the two kinds of flesh. Thus, while 20 grams of beef 
contain on an average 5.1 grams of solid matter, the flesh of frogs' 
legs contains but 3.5 grams. It is evident from this example, then, 
that a determination of the total solid matter is necessary in each 
species of flesh ; but even when that is done, and corrections made 

» Zeitschrift fiir Biologic 19, 130. « Ibid. p. 140. 



accordingly, we have found a decided difficulty in filtering the diges- 
tive mixtures. The undissolved residue of the fish is so gelatinous 
that it is next to an impossibility to wash it entirely free from peptones. 
We, therefore, decided to work with the filtrate, and after several 
trials by precipitating the dissolved albumin with tannic acid, accord- 
ing to the method of Johnson," and also by determining the specific 
gravity of the fluid after filtration, we finally adopted the follow- 
ing method, which has proved quite satisfactory. After the gastric 
juice has been allowed to act for the requisite length of time on 
the 20 grams of flesh the mixture is cooled to 20° C, and then 
diluted to 250 cc, in a graduated flask, with distilled water. After 
being thoroughly mixed it is filtered on a dry filter, and then 50 cc. 
or one-fifth of the entire mixture, is transferred by a pipette to a 
small weighed dish, and to it are added 5 cc. of a standard solution 
of sodium carbonate of such strength as exactly to neutralise the 
acid present. The fluid is then evaporated to dryness on the water- 
bath, and finally dried at 110° C. until of constant weight. In 
order that the results obtained may express the absolute amounts 
of matter dissolved by the gastric juice, it is necessary to carry 
on control experiments with the gastric juice itself This is also 
desirable as a proof of the uniform strength of the gastric juice. 
Thus in each series of experiments 200 cc. of the standard juice 
were warmed at 38-40° C. for 22 hours, so that all albumin con- 
tained in the pepsin could be converted into peptone, then diluted 
to 250 cc. and 50 cc, neutralised, evaporated, and dried as already 
described. This residue, subtracted from the weight of the residue 
left by the evaporation of the 50 cc. of the digestive mixtures, mul- 
tiplied by 5, gives quite accurately the amount of matter (peptones 
and intermediate products together with some salts) dissolved from 
the 20 grams of flesh. Theoretically there are one or two minor 
objections to this method, the most noticeable, perhaps, being the 
variable amount of undigested residue suspended in the fluid diluted 
to 250 cc. The amount of space, however, occupied by this matter 
is not large, and its influence on the accuracy of the method not 
great. Again, by the evaporation of the peptones, and drying at 
110° C, there is doubtless some slight oxidation, but still it cannot 
be great, as the dried residue is soon brought to a constant weight. 
While the method seems longer than to weigh the undissolved 
residue, it is by far more accurate, and in the end we believe shorter. 

' Bulletin de la Societe chimique de Paris 23, 40. 



The fish experimented" with were obtained at a local market, and 
while always quite fresh and in good condition, we had no means 
of knowing how long they had been out of water. 

In all of the experiments the flesh was cooked by steaming, 
unless otherwise expressed. 

Tables I-V give all the data of the experiments, while table VI 
contains the average amounts digested of each sample of cooked 
flesh, together with the relative digestibility as compared with 
cooked (steamed) beef, taken as loo. 

A glance at the results of the control analyses of the gastric 
juice alone plainly shows the constancy of its composition. The 
strength and activity of the digestive mixture, moreover, is easily 
seen from the amount of blood fibrin (Table IV) dissolved by 200 
cc. of the fluid ; an amount far in excess of the fish or beef dis- 
solved by the same quantity of fluid. 

The results of the analyses show plainly that the method adopted 
is as good as could be expected, for it must be remembered that 
the two results obtained from each sample of flesh are not merely 
from duplicated analyses, but from duplicated digestions as well, 
and in these, extending as they do over 22 hours, with slight vari- 
ations in temperature and agitation, small differences are to be 
expected. The very great divergence noticed, however, in the re- 
sults obtained from different samples of the same species of flesh 
show at once that there are other conditions, such as age, etc., 
which affect the digestibility of the flesh more or less, so that, in 
order to obtain results from which to draw strict generalisations, it 
would be necessary to experiment with fish of different species, of 
like age, sex, and reared under like conditions. As examples of 
this we have the very divergent results from two samples of veal, 
and also of two blue fish (88.69 and 73.44). As direct evidence 
that age, sex, etc., do exert a modifying influence on the digesti- 
bility of flesh, wc have three experiments on the flesh of the lobster; 
one with a small young lobster, a second with a large female, and a 
third with a large male of the same species. The duplicate digestions 
gave fairly concordant results ; the average relative digestibility 
being for the young specimen 87.81, for the large female 79.06, and 
for the male 69.13. This shows plainly some modifying influence 
in the flesh itself In composition, so far as the solid matter is con- 

'For composition of the fish experimented with, see " Zur Chemie der Fische," Berichte der 
deutsch. chem. Gesell. 16, 1839, by Prof. W. O. Atwater. 



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12 



Table VI. 

Average results from each sample of cooked flesh. 



Beef 

Beef 

Beef 

Veal 

Veal 

Mutton 

Lamb 

Spring chicken (light meat) 

" (dark meat) 

Lake White (Coregonus clupeiformis] 

Shad (Clupea sapidissima) 

" " (light meat 

" " (dark meat) 

Salmon (Salmo salar) 

Tautog (Hiatula onitis) 

Porgee (Sparus chrysops) 

Blue fish (Pomatomus saltator) 

Mackerel (Scomber scombrus) 

Halibut (Hippoglossus vulgaris) 

Flounder (Paralichthys dentatus) 

Sea Bass (Serranus atrarius) , 

Pike (Esox lucius) 

Haddock (Gadus aeglifinus) 

Herring (Clupea harengus) , 

Striped Bass (Roccus lineatus) 

Red Snapper (Lutjanus Blackfordi).., 
Trout, brook (Salvelinus fontinalis).. 

Sea Robin (Prionotus palmipes) , 

White Perch (Roccus Aniericanus)... 

Fresh Cod (Gadus callarias) , 

Weak fish (Cynoscion regale) 

Yellow fish(Perca Americana) 

Eel (Anguilla rostrata) 

Window Pane (Bothus maculatus) ... 
Flat fish (Pleuronectidae Americanus 
Lobster (young) 

" (large female) 

" (large male) 

Crab 

Frogs' legs 



19 



5.024 
5.206 

5-138 
4.992 
4.858 
6.168 
5-974 
5-32S 

5-340 
5.112 
6.266 
6.076 
6.526 
6.212 
6.300 
4.120 

4-512 
3.968 
3.892 
5.102 
4.056 
4.608 

4-234 
3.926 
3.648 
4.89S 
4.164 
4.418 
3.916 
4-374 
3-93^ 
3.658 
3-956 
3.624 

4-356 
3-674 
3-430 
4-350 
4.258 
4.152 
4.714 
3-572 



4.1607^ 

4-1167 \ 

3.S6ioJ 

4.1742 ) 

3.5052) 

3-7287 

3-5580 

3-5090 

3.4160 

3-8350 
3-6455 
3-9352 
3 5332 
3-7345 
3-6335 
3.5660 

3-5215 
3-5885 
2.9717 

3-4895 
3.4600 

3-4525 
3-3995 
3-3582 
3-3382 
3-3317 
3.2770 

3-3040 
3-1745 
3-1572 

2-9515 
2.9292 
2.9180 

2.9037 
2.9062 
2.8927 
2.7065 

3-5532 
3.1990 
2.7960 
2.7165 
3-2535 



i e i ^ 

o o J? ^ 
'^ ^ o -^ 

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Z=IOD.OO 



= 94.89 
92.15 

87-93 
86.72 
84.42 
94.78 
90.09 
97.25 
87-32 
92.29 
89.80 
88.13 
87.03 

88. 69 

73-44 
86.24 

85-51 
85-32 
84.01 
82.99 
82.50 
82.34 
80.99 
81.65 

78.45 
78.03 
72.94 

72-39 
72.1 1 
71.76 
71.82 
71.49 
66.89 
87.81 
79.06 
69.13 

67-13 
80.46 



13 

cerned, there was no appreciable difference in the three samples. 
Bearing in mind, however, these possible variations, it is very evident 
from our results that the average digestibility of fish flesh is far 
below that of beef similarly cooked. In but two instances, in the 
case of shad and lake white, does the digestibility of fish flesh 
approach that of beef, although, from the average of our experi- 
ments, several are as easily digestible as mutton, lamb and chicken. 

Pavy' states that fish with white flesh, such as the whiting, etc., 
are less stimulating and lighter to the stomach, or more easy of 
digestion, than fish with more or less red flesh, as the salmon. Our 
experiments confirm this statement so far as digestibility is con- 
cerned. Thus the average digestibility of the salmon and trout is 
considerably below the average of the more digestible white fish. 
The difference between the digestibility of the light and dark meat of 
the same fish is somewhat striking, as in the case of the shad, where 
the digestibility of the former was found to be 97.25, as compared 
with beef, while the dark flesh was 87.32. A similar difference, 
though very much smaller, is to be noticed between the light and 
dark meat of the chicken. 

This difference in digestibility is in part due, without doubt, to 
the amount of fat present, for, as Pavy states, in the flesh of white 
fish there is but little fat, it being accumulated mainly in the liver 
of the animal, while in red fish there is more or less fatty matter 
incorporated with the muscular fibres. For a similar reason eels, 
mackerel and herring are, according to Pavy, less suited to a deli- 
cate stomach than some of the white fish, and our experiments 
show that in digestibility two of them stand below the more diges- 
tible white fish ; mackerel, however, from our single experiment 
with the white portion of the flesh, showed a comparatively high 
digestibility. In all of our experiments, however, with white fish, we 
rejected the outer layer of dark flesh, except in the case of the shad. 
The varying differences in digestibility are not to be considered as 
due wholly to differences in the amount of fat in the flesh ; thus the 
flesh of fresh cod contains but litde fat, and yet it is one of the most 
indigestible of the white fish experimented with. This agrees with 
Pavy V experience " that it is a more trying article of food to the 
stomach than is generally credited." Again Pavy^ makes the fol- 
lowing statement, based on his experience in fish dietetics, " of all 
fish, the whiting may be regarded as the most delicate, tender and 

1 On food and dietetics. Amer. ed., 1874, p. 171. ^ Loc czt.p. 173. ^Loc. cit. p. 172. 



H 

easy of digestion." " The haddock is somewhat closely allied, but 
is inferior in digestibility," while " the flounder is light and easy of 
digestion, but insipid." With all these statements our results agree 
perfectly, assuming the lake white of our experiments to be analo- 
gous to the English whiting. 

Maly,' in speaking of the digestive processes in the living stom- 
ach, says that raw flesh is more slowly digested than cooked, 
probably for the reason that with dilute acids the coagulated 
albumin of cooked flesh is more easily converted into acid albumin. 
Likewise, that the flesh of young animals is more rapidly digested 
than that of older, while fat flesh is but slowly attacked, as the 
melted fat surrounds the muscle fibres. With reference to the 
first of these statements, Jessen ^ found, by experimenting with per- 
fectly lean beef of known age, that he had only a small undigested 
residue in an artificial digestion of the raw beef, but with the same 
amount of partially boiled beef a much larger amount remained 
undigested, and when thoroughly boiled a still larger residue was 
found. Taking the amount of undigested residue as a measure of 
the digestibility, the proportion with the same sample of beef was 
as follows: raw beef lOO, partially boiled 167, thoroughly boiled 
317. The gastric juice employed by Jessen, however, could hardly 
be considered as made up of a dilute acid, containing, as it did, 
2.5 and 5.0 per cent, of concentrated hydrochloric acid. 

In our own experiments, with a gastric juice containing but 0.2 
per cent, of pure hydrochloric acid, positive results were obtained 
as follows : 

isi sample of Beef. zd sample of Beef 

Raw. Cooked (steamed). Raw. Cooked (steamed). 



Amount digested from 










20 grams. 


4.0792 


3.8610 


4-3785 


4.1607 


Relative proportion. 


1 00.0 


94-65 


lOO.O 


95-04 



The difference here, then, is not so great, though sufficiently pro- 
nounced to indicate plainly the influence of cooking. 

A similar experiment with a sample of blue fish gave a like 
result : 

Amount digested from 20 grams. 
Relative proportion, 

' Hermann's Handbuch der Physiologic 5, iii. ' Zeitschrift fiir Biologic 19, 128 



Raw. 


Cooked (steamed), 


3.7617 


3-5885 


1 00.0 


95-39 



15 

With the raw beef, however, digestion was so near complete 
that a second experiment was tried with a larger quantity, as fol- 
lows : 

From 20 grams. From 30 grams. 

Amount digested (raw beef), 4-3785 5-76 lo 

Relative proportion, loo.o 131 -57 

This would make the relativedigestibility of cooked (steamed) 
and raw beef as loo.o : 142.38, a difference nearly as great as that 
found by Jessen between raw and partially boiled beef. It is plain, 
then, that the digestibility of raw beef is considerably greater than 
cooked. 

Whether the relative digestibility of raw and cooked fish, given 
above, would be changed by increasing the amount of flesh added, 
we cannot say. It is a point of little importance, but from the fol- 
lowing experiment with cooked fish it probably would make but 
little difference. This experiment with cooked sea bass was tried 
mainly to ascertain the quantity of flesh best adapted to 200 cc. of 
our standard gastric juice : 

From 20 grams. From 30 grams. From 40 grams. 

Amount digested, 3-3995 3-2325 2.5200 

Relative proportion, loo.o 95.08 74-12 

In this case, increasing the amount of material plainly diminishes 
the digestive action. 

With regard to the second statement of Maly's, above quoted, 
our experiments tend to show that, in some instances at least, the 
flesh of younger animals is less easily digestible than that of older 
animals of the same species. This is well illustrated in the greater 
digestibility of mutton as compared with lamb. 



'Tfn'.^i i w iii 



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